Thread: Holley Jets, WHAT the heck???

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A spread of 71 front/82 rear is normal for vacuum secondary carbs. You are looking at essentually two-two barrel carbs fixed in tandem....

The front bores being smaller use smaller jet size, the rears are MUCH larger, hence the larger jet size.

Mechanical secondary carbs usually run 4-barrels the same size, with close to the same jets. As you noted: stagger jetting in this case is a result of manifold/distribution differences.

When doing jetting changes, reading the sparkplugs is complicated by the lack of lead in the fuel. That was what colored the porcelain so well. True: you must do at least 20 runs to get some type of color to show.

But, When doing seat-of-the-pants tuning, Disconnect the secondary link so the secondaries dont work. Warm up engine and do your 1-2 runs. Rich? (rich is easy to tell due to the black plug color and sluggish acceleration). Drop 2 jets and run again. Better, but rich still, drop 2. Keep doing until the plugs clean up.

Reconnect the link and start all over on the secondaries. Measure your idling vacuum. install a power valve 3 sizes under what your vacuum reading was.

I could go on for hours on carb tuning. Keep us informed as to your progress. MIZ

Originally Posted by mizlplix

A spread of 71 front/82 rear is normal for vacuum secondary carbs. You are looking at essentually two-two barrel carbs fixed in tandem....

The front bores being smaller use smaller jet size, the rears are MUCH larger, hence the larger jet size.

Mechanical secondary carbs usually run 4-barrels the same size, with close to the same jets. As you noted: stagger jetting in this case is a result of manifold/distribution differences.

When doing jetting changes, reading the sparkplugs is complicated by the lack of lead in the fuel. That was what colored the porcelain so well. True: you must do at least 20 runs to get some type of color to show.

But, When doing seat-of-the-pants tuning, Disconnect the secondary link so the secondaries dont work. Warm up engine and do your 1-2 runs. Rich? (rich is easy to tell due to the black plug color and sluggish acceleration). Drop 2 jets and run again. Better, but rich still, drop 2. Keep doing until the plugs clean up.

Reconnect the link and start all over on the secondaries. Measure your idling vacuum. install a power valve 3 sizes under what your vacuum reading was.

I could go on for hours on carb tuning. Keep us informed as to your progress. MIZ

What??,a vacuum secondary carb does not use a metering block or rear jets un-less he has put on a conversion kit on it.
Holley use to have a carb much like the 3310 that had a secondary metering block along with the rear jets.
I think it flowed in the 780 area.
If this is a double pump carb I would first start by "squaring up" the jets.

Erik, what do you mean by "squaring up the jets"?

Originally Posted by NTFDAY

Erik, what do you mean by "squaring up the jets"?

I mean start with the same size jet in the front as you have in the back.

Thanks for the explanation, I don't believe I'd heard that before.